Surface Plasmon Polaritons (SPP) or Surface Plasmons are electromagnetic waves that can propagate along the interface between two media, such as at the interface between a metal and a dielectric material. They correspond to oscillations of electrons at the interface between the materials, such as those of electrons excited by photons. Surface plasmon resonance can be observed by measuring the reflectivity of p-polarized light of a given wavelength at a metal-dielectric interface for a varying angle of incidence. Alternatively, surface plasmon resonance can be observed by measuring the reflectivity of p-polarized light for a given angle of incidence at a metal-dielectric interface and a wavelength of the p-polarized light can be varied.
Surface plasmons present at the interface between, for example, a metal and a dielectric layer may be very sensitive to refractive index changes at the interface. The principle of surface-plasmon-resonance-based chemical sensing involves providing a thin layer of chemically active material, which may be called a sensing layer, on the metal surface at which the surface plasmons are excited. The excitation of the surface plasmon resonance is directly related to the interface properties. Changes in the sensing layer brought about by the presence of an analyte result in changes in the excitation angle or resonant wavelength of the surface plasmon resonance. By monitoring the excitation angle or the resonant wavelength, an analyte concentration may be determined.
The working principle of a surface-plasmon-resonance-based sensor can, for example, be based on the incident angle dependence of the surface plasmon resonance. Such an approach requires a prism coupler and may require rotatable equipment to obtain a larger range of incident angles. However, this type of sensor involving alignment and discrete components is not suitable for cheap mass production. Further, such sensors may suffer from the influence of vibrations due to the use of discrete components such as micro-objective lenses, lasers and photodetectors.
Another approach is wavelength spectroscopy surface plasmon resonance, in which a resonant wavelength shift of surface plasmon resonance is monitored. In this approach the need for a prism is avoided, and the size of the sensor can be smaller However, it requires a multi-wavelength source and a spectrometer, which can be expensive.
Surface plasmon resonance has been applied to different sensor applications such as, for example, for label-free sensing in biochemistry applications. However, commercially available surface plasmon resonance sensors are large in size, costly, not suitable for real-time monitoring, and require trained personnel to carry out the bio-analysis. By comparison, real-time monitoring with optical sensors involves smaller size sensors using lower cost transducers and lower power consumption.
In U.S. Pat. No. 6,424,418, Kawabata proposes a surface plasmon resonance sensor comprising a Vertical Cavity Surface Emitting Laser (VCSEL) and a Charge Coupled Device (CCD) array built on a common substrate. A cylindrical lens is provided above the laser to expand the laser light, and a thin metal film is provided for reflecting the laser light towards the CCD array. When light is emitted from the laser and impinges on the metal film, surface plasmon resonance can be induced at an incident angle which satisfies the surface plasmon resonance excitation condition. The surface emitting laser, metal film and CCD array are positioned such that the change in intensity of light reflected by the thin metal film, caused by the surface plasmon resonance, can be measured by the CCD array. However, the sensor is relatively large. Moreover, the technology involved requires integration between a Silicon CCD and VCSEL III-V material. As a single sensor has a limited measurement range, a VCSEL array and a two-dimensional CCD array may be needed.
In Japanese Patent Application Pub. No. JP 2005 308658, Takaaki discloses a compact and low cost surface plasmon resonance sensor. A VCSEL is used as a light source, and a sensor part is provided at the light-emitting surface of the VCSEL, the sensor part comprising a dielectric layer having an upper surface with a grating made by concentric recessions and protrusions. A metal film is provided on the surface of the dielectric layer, and receptors are fixed to the surface of the metal film. The metal film has an opening at its center part and an optical fiber probe is provided in the vicinity of the opening to detect leaking near-field light. In the absence of an analyte, surface plasmon resonance occurs in the sensor part, and facilitates the transmission of light through the central opening of the metal structure. Upon adsorption of the analyte, surface plasmon resonance no longer exists in the sensor part, and the intensity of the leaking light changes. This change in intensity of the transmitted power is detected by means of the optical fiber probe. In this approach, however, external detection (e.g., using the optical fiber probe) is needed, requiring additional alignment. Further, there exists a trade-off between the sensor sensitivity and the optical power transmission. A higher power transmission can be obtained with a larger central opening in the metal structure, but this leads to a broader spectral width, which limits the sensor sensitivity. On the other hand, a narrower spectral width and thus a higher sensitivity can be obtained with a smaller central opening, but this results in a very low optical power transmission.